As is well known, the electrostatic coating method comprises applying a high-voltage direct current between an object and a spray-coating apparatus so as to form a line of electrostatic force therebetween and thereby cause a spray of charged paint to adhere to the surface of the object along the electrostatic force line. The paint consumption loss is far lower (about one-half) as compared to paint loss using conventional non-electrostatic spray-coating methods. Another advantage of electrostatic spray-coating method is that a coating film can be formed uniformly over the entire object, including shadows. The electrostatic coating method is, therefore, widely employed. When the electrostatic coating method is employed for coating a non-conductive substance such as a non-conductive plastic, a conductive agent is typically applied as a primer to the object in order to render it electrically conductive. That is, the conductive agent is applied to the surface of the non-conductive object so as to form an electrically conductive film. The electrically conductive film is thereafter charged so as to perform the electrostatic coating.
The conductive agents usable for the above-described purposes typically include, for example, various amphoteric, conductive organic substances such as quaternary ammonium salts, as well as cationic, anionic and nonionic conductive organic substances. They are usually diluted with a suitable solvent and the solution is then applied to the surface of the object to form an extremely thin primer layer prior to the electrostatic coating operation. However, known conductive agents are prepared mainly for the purpose of increasing the electrostatic efficiency but the effects thereof on the adhesion of the electrostatic coating film to the coated object have rarely been examined. In this regard, it has been found that a number of problems exist when one attempts to electrostatically coat a highly crystalline plastic having very low polarity and poor adhesion (hereinafter sometime referred to as "problematic plastics") such as polyacetal and polyester resins. Although it is indispensable that a conductive film be applied as the primer layer when such problematic plastics are electrostatically coated, use of conventional conductive agents impart poor adhesion properties to such plastics. Even when the affinity of an electrostatic paint for the plastic is improved by suitably adjusting the paint composition, a film having a conductive layer is easily peeled from the plastic. Thus, satisfactory paint adhesion has not been obtained using conventional conductive agents in connection with problematic plastics, such as polyacetal and polyester resins.
It is therefore an object of the present invention to provide a conductive agent capable of effectively performing its intended essential functions--that is, exhibiting excellent "wettability" for problematic plastics in addition to forming a primer layer which firmly adheres an electrostatic coating film to the surface of an object formed of such problematic plastics. Another object of the present invention is to provide electrostatically spray coated moldings in which the electrostatic coating layer is firmly adhered via such a conductive agent.
According to the present invention, these objects are provided by a conductive agent for electrostatic coating which comprises as the main constituents:
(A) a polyurethane, and PA1 (B) a conductive inorganic fine powder and/or a conductive organic substance.
Electrostatically spray coated plastic moldings which employ the above-noted conductive agent exhibit excellent paint-adhesion properties and thus overcomes many of the problems associated with the electrostatic spray-coating of problematic (crystalline) plastics.